(1) Field of the Invention
The present invention relates to an apparatus for butt welding steel strips by means of a laser beam (hereinafter referred to as "laser butt welding apparatus") in a steel strip-processing line.
(2) Description of the Prior Art
In a steel strip steel processing line, steel strips are joined together at an entry section of the line, and fed to a latter stage processing and continuously treated and produced. The steel strips have been conventionally jointed by a seam welding machine, a flash butt welding machine, an arc welding machine such as TIG arc welding machine. These welding methods have their own peculiar characteristics respectively. The seam welding is characterized in that the welding is possible at a high speed in a short time, but the thickness of the welded portion becomes 180-130% of that of the base metal because the steel strips are welded by piling their edges one upon another. Therefore, the welded portion can not be rolled in a coil by taking the problem such as buckling into account. If a scale is present on the surface of the steel strip as in the case of a hot-rolled steel strip or the like, this method has the defect that the surface must be ground prior to the welding. Further, in the case of the welding of thin steel strips, since the strips are pressed by means of electrode wheels, this method has the drawback that the profile of junction and the vicinity thereof are deteriorated.
The flash butt welding can make the welding time shorter than the seam welding because the steel strips are welded at one time over their whole width in the former method, but thin steel strips are likely to be bent due to upsetting step, so that this method has the defect that it is applicable to the relatively thick steel strips of not less than 1.6 mm, but is not applicable to the thin steel strips. It also has the defect that it is inapplicable to special steels such as silicon steel, stainless steel, high carbon steel because of oxidation, a large heat input and the coarse grain growth at the welded portion and heat affected zone when the flashing is effected. As a result, the use of flash butt welding is generally restricted to the welding of the low carbon steel strips of not less than 1.6 mm in thickness.
According to TIG arc welding, it is possible to improve the quality of the welded portion through addition of a filler wire or the like, and therefore, this method can be used for stainless steel. However, since the heat input is large, it has the defect that it is inapplicable to the material such as the silicon steel in which the coarse grains are likely to grow. Further, it also has the defects that the welding speed is slow, and the welding time is long.
As mentioned above, although the conventional seam welding, flash butt welding, TIG arc welding and the like have been used by utilizing the respective characteristics, they can not be applied to the silicon steel or the like.
As such a welding machine, the inventors have proposed a laser butt welding apparatus.
In the case of the laser butt welding of the steel strips, particularly made of high Si steel (Si.gtoreq.0.8%) and high carbon steel (C.gtoreq.0.3%), it has been found that the welding strength is enhanced by the provision of a filler wire at the butted edges of the steel strips to be welded together.
In such a welding method, it is common practice that the welding is done at a beam diameter of about 0.2-0.8 mm with respect to the gap of 0.05-2 mm between the butted edges of the steel strips. Before carrying out the butt-welding, the edge portions of the steel strips are cut by means of a shear, but there is the possibility that a uniform lateral gap between the edges of the steel strips is not formed, or the track of the laser beam is not coincident with the center line of the gap due to a poor precision of the shear.
This phenomenon will be elucidated in more detail with reference to FIG. 27(a).
FIG. 27(a) illustrates the shape of the gap formed when the edges of the steel strips are butted with each other. Reference numerals Sa and Sb are preceding and trailing steel strips, and reference numerals 702 and 703 are the track of the laser beam and the center line of the gap between the butted edges of the steel strips respectively. At this state, the welding is carried out by running the laser beam along the straight line 702 in the transverse direction of the steel strips, while being kept at a constant position in the longitudinal direction without being adjusted to be longitudinally displaced, and by supplying a filler wire at a constant rate. But, the fused filler wire is lacking at the portions a and c, while to the contrary, the fused filler wire is in excess at the portions b and d. Particularly, the gaps at the positions a and c are not completely filled with the fused filler wire and the amount of the filler wire fused is not sufficient and the welding strength is poor. With respect to the positions b and d having the excessively fused filler wire, such a method is economically unfavorable.
Thus, there has been demanded a welding apparatus which can weld the special steels such as silicon steel or the like.